This invention relates to an apparatus for damping vibrations created by the movement of a helicopter rotor.
The connection, to the rotor hub, of pendulous members which swing under the effect of the centrifugal force, in order to damp or eliminate the vibration forces perpendicular to the rotor axis of rotation, called coplanar forces, is known in the prior art. By adjusting the dimensions (pendulum swing radius and distance between the axis of rotation and the center of gravity of the pendulous member), it is possible to adjust the natural swing angular frequency to an angular frequency at which it is desired to damp the vibrations. With respect to these coplanar forces, it is desirable to damp the vibrations of angular frequency (b-1).OMEGA. and (b+1).OMEGA., where b denotes the number of rotor blades and .OMEGA. denotes the rotor angular speed of rotation. The aforementioned pendulum damping systems, such as described in U.S. Pat. No. 3,035,643, are rather limited with respect to their natural angular frequency. They are accordingly mainly suitable for helicopter rotors having a small number of blades. These limitations can be alleviated by utilizing a bifilar connection between the pendulous members and the hub (see U.S. Pat. No. 3,540,809). While this bifilar system is theoretically very interesting, it nevertheless presents a number of disadvantages in practice. Thus, the structural strength, in outdoor operating conditions, of the tracks of the bifilar connections poses a serious problem. The solutions adopted to solve this problem (for example, stainless steel, bearings, frequent grease lubrication) are not always convenient, and do not guarantee sufficient reliability (scaling, failure, . . . ), making them unsatisfactory. Furthermore, the simultaneous mounting of two assemblies, one for (b-1).OMEGA. and the second for (b+1).OMEGA., is penalizing in terms of weight and size. The structural ratio, that is, the ratio of the movable mass to the total mass of the system, is finally mediocre.
An object of the present invention is to provide another pendular damping structure for helicopter rotor, which avoids at least some of the aforementioned limitations.